Copyright  Oracle Corporation, All rights reserved. 6 Subqueries

6-2 Copyright  Oracle Corporation, All rights reserved. Objectives After completing this lesson, you should be able to do the following: Describe the types of problems that subqueries can solve Define subqueries List the types of subqueries Write single-row and multiple-row subqueries After completing this lesson, you should be able to do the following: Describe the types of problems that subqueries can solve Define subqueries List the types of subqueries Write single-row and multiple-row subqueries

6-3 Copyright  Oracle Corporation, All rights reserved. Using a Subquery to Solve a Problem “Who has a salary greater than Jones’?” “Which employees have a salary greater than Jones’ salary?” Main Query ? “What is Jones’ salary?” ? Subquery

6-4 Copyright  Oracle Corporation, All rights reserved. Subqueries The subquery (inner query) executes once before the main query. The result of the subquery is used by the main query (outer query). The subquery (inner query) executes once before the main query. The result of the subquery is used by the main query (outer query). SELECTselect_list FROMtable WHEREexpr operator (SELECTselect_list FROMtable);

6-5 Copyright  Oracle Corporation, All rights reserved SQL> SELECT ename 2 FROM emp 3 WHERE sal > 4 (SELECT sal 5 FROM emp 6 WHERE empno=7566); Using a Subquery ENAME KING FORD SCOTT ENAME KING FORD SCOTT

6-6 Copyright  Oracle Corporation, All rights reserved. Guidelines for Using Subqueries Enclose subqueries in parentheses. Place subqueries on the right side of the comparison operator. Do not add an ORDER BY clause to a subquery. Use single-row operators with single- row subqueries. Use multiple-row operators with multiple-row subqueries. Enclose subqueries in parentheses. Place subqueries on the right side of the comparison operator. Do not add an ORDER BY clause to a subquery. Use single-row operators with single- row subqueries. Use multiple-row operators with multiple-row subqueries.

6-7 Copyright  Oracle Corporation, All rights reserved. Types of Subqueries Single-row subquery Main query Subquery returns CLERK Multiple-row subquery CLERKMANAGER Main query Subquery returns Multiple-column subquery CLERK 7900 MANAGER 7698 Main query Subquery returns

6-8 Copyright  Oracle Corporation, All rights reserved. Single-Row Subqueries Return only one row Use single-row comparison operators Return only one row Use single-row comparison operators Operator = > >= < <= <> Meaning Equal to Greater than Greater than or equal to Less than Less than or equal to Not equal to

6-9 Copyright  Oracle Corporation, All rights reserved. Executing Single-Row Subqueries CLERK 1100 ENAME JOB MILLER CLERK ENAME JOB MILLER CLERK SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4(SELECT job 5 FROM emp 6 WHERE empno = 7369) 7 AND sal > 8(SELECT sal 9FROMemp 10WHEREempno = 7876);

6-10 Copyright  Oracle Corporation, All rights reserved. Using Group Functions in a Subquery 800 ENAME JOB SAL SMITH CLERK 800 ENAME JOB SAL SMITH CLERK 800 SQL> SELECTename, job, sal 2 FROMemp 3 WHEREsal = 4(SELECTMIN(sal) 5FROMemp);

6-11 Copyright  Oracle Corporation, All rights reserved. HAVING Clause with Subqueries The Oracle Server executes subqueries first. The Oracle Server returns results into the HAVING clause of the main query. The Oracle Server executes subqueries first. The Oracle Server returns results into the HAVING clause of the main query. 800 SQL> SELECTdeptno, MIN(sal) 2 FROMemp 3 GROUP BYdeptno 4 HAVINGMIN(sal) > 5(SELECTMIN(sal) 6FROMemp 7WHEREdeptno = 20);

6-12 Copyright  Oracle Corporation, All rights reserved. What Is Wrong with This Statement? ERROR: ORA-01427: single-row subquery returns more than one row no rows selected ERROR: ORA-01427: single-row subquery returns more than one row no rows selected SQL> SELECT empno, ename 2 FROM emp 3 WHERE sal = 4(SELECT MIN(sal) 5FROM emp 6GROUP BY deptno); Single-row operator with multiple-row subquery

6-13 Copyright  Oracle Corporation, All rights reserved. Will This Statement Work? no rows selected Subquery returns no values SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4(SELECTjob 5FROMemp 6WHEREename='SMYTHE');

6-14 Copyright  Oracle Corporation, All rights reserved. Multiple-Row Subqueries Return more than one row Use multiple-row comparison operators Return more than one row Use multiple-row comparison operators Operator IN ANY ALL Meaning Equal to any member in the list Compare value to each value returned by the subquery Compare value to every value returned by the subquery

6-15 Copyright  Oracle Corporation, All rights reserved. Using ANY Operator in Multiple-Row Subqueries EMPNO ENAME JOB MARTIN SALESMAN 7521 WARD SALESMAN EMPNO ENAME JOB MARTIN SALESMAN 7521 WARD SALESMAN SQL> SELECT empno, ename, job 2 FROM emp 3 WHERE sal < ANY 4(SELECTsal 5 FROMemp 6WHEREjob = 'CLERK') 7 AND job <> 'CLERK';

6-16 Copyright  Oracle Corporation, All rights reserved. Using ALL Operator in Multiple-Row Subqueries EMPNO ENAME JOB KING PRESIDENT 7566 JONES MANAGER 7902 FORD ANALYST 7788 SCOTT ANALYST EMPNO ENAME JOB KING PRESIDENT 7566 JONES MANAGER 7902 FORD ANALYST 7788 SCOTT ANALYST SQL> SELECT empno, ename, job 2 FROM emp 3 WHERE sal > ALL 4 (SELECTavg(sal) 5 FROMemp 6GROUP BYdeptno);

6-17 Copyright  Oracle Corporation, All rights reserved. Summary Subqueries are useful when a query is based on unknown values. SELECTselect_list FROMtable WHEREexpr operator (SELECT select_list FROM table);

6-18 Copyright  Oracle Corporation, All rights reserved. Practice Overview Creating subqueries to query values based on unknown criteria Using subqueries to find out what values exist in one set of data and not in another Creating subqueries to query values based on unknown criteria Using subqueries to find out what values exist in one set of data and not in another

6-19 Copyright  Oracle Corporation, All rights reserved.

6-20 Copyright  Oracle Corporation, All rights reserved.